1. Field of the Invention
The present invention relates to sheet transport apparatuses, and to document reading apparatuses and image forming apparatuses in which these are provided.
2. Description of the Related Art
It is known that conventional sheet transport apparatuses provided in image forming apparatuses and document reading apparatuses use, as a detection means that detects a presence/absence of a sheet such as a recording paper or a document or the like transported on a sheet transport path, an optical sensor that has a light-emitting portion and a light-receiving portion and detects the presence/absence of the transported sheet according to a magnitude of a light amount at the light-receiving portion received as light from the light-emitting portion.
With these optical sensors, the amount of light emitted from the light-emitting portion (for example, a light-emitting diode (infrared LED) that emits infrared beams) sometimes drops due to change over time. When this happens, even if a sheet is being transported (there is a sheet), a detection error may occur that a sheet is not being transported (there is no sheet).
In consideration of the drop in amount of emitted light due to change over time, it is conceivable to raise the light amount of the light-emitting portion from the beginning, but in this case the drive current to the light-emitting portion increases, thereby incurring a reduction in the life of the light-emitting portion.
In contrast to this, JP 2003-267589A discloses an image forming apparatus in which a determination is performed as to whether or not a transport count number of sheets has reached a predetermined value so as to adjust the light amount of the light-emitting portion.
However, in this image forming apparatus, the light amount of the light-emitting portion is adjusted when the transport count number of sheets has reached the predetermined value, and therefore in a case where the light amount of the light-emitting portion has dropped before the transport count number of sheets has reached the predetermined value, a sensor detection error occurs undesirably.